The six-minute walk test (6MWT) is broadly used in clinical research, yet there are many unanswered questions about the test. Unlike maximal exercise testing (CPX), 6MWT is conventionally considered a sub-maximal exercise test and cannot delineate physiologic mechanisms for limitation. One consequence is the inability to account for confounding factors such as motivation. Despite the limitation, 6MWT physiologic parameters approach maximal values. Consequently, metabolic measurements during a 6MWT, afforded by recent technological developments, can confirm this hypothesis and lead to development of an exercise test that combines the advantages of 6MWT with those of CPX. Unfortunately, methodologic variations threaten the external validity of 6MWT. We will, therefore, study one important, previously unstudied variable - the impact of a straight vs. circumferential walking track - upon walk distance. Moreover, the physiologic strategies subjects use during 6MWT are largely unknown. Consequently, we will quantify these strategies by collecting physiologic data throughout the 6MWT. Then, we will explore differences in these physiologic strategies across serial walks and between the two track configurations.